Clegg's argument that any spare money should be used to support "all working families" is a powerful dividing line with the Conservatives.

The decoration of a wedding cake is seen at the wedding fair on January 8, 2012 in the western German city of Essen. Photograph: Getty Images.

After three years in government, it is only now that David Cameron is talking seriously about introducing a married couples' tax allowance, promising to announce plans "very shortly". The proposal was included in the 2010 Conservative manifesto and the Coalition Agreement (which gave the Lib Dems the right to abstain), so what's taken him so long? The reason for the delay, I suspect, is that influential Tories, most notably George Osborne (one of the most socially liberal MPs), recognise that it is a profoundly flawed measure.

As Nick Clegg observed at his press conference this morning, a policy that "basically says to people who are not married: you will pay more tax than people who are married" will seem odd to many voters. Clegg's argument that a spare £550m would be better spent "on all working families" is a powerful dividing line with the Tories.

It's true that a YouGov poll in January found that 53 per cent of the public support the introduction of a tax allowance, with just 36 per cent opposed, but this total is likely to fall when voters find out that less than a third of married couples would benefit. As outlined in the Tory manifesto, only those couples in which one member is not using all of their personal tax allowance would gain (they could transfer £750 of their unused allowance to their spouse), with higher-rate taxpayers excluded. The policy could, of course, be redesigned so that all or most married couples benefit but this, not least for the fiscally conservative Osborne, would be prohibitively expensive. Clegg's argument that any spare money would be better used on raising the personal allowance is likely to resonate.

In his recent GQ article, Andy Coulson described the perception that David Cameron does not like single parents as "electoral halitosis", but this policy unambiguously discriminates against them. Among those who also don't gain from the policy, as Don't Judge My Family notes, are widows and widowers, people who leave abusive relationships and working couples. The policy will also alienate the young, socially liberal voters that the Tories need to win over if they are to ever secure a majority (18-24-year-olds opposed the policy by 40-36 in the YouGov poll).

In reaffirming his support for an allowance, Cameron may enhance his standing with his party and with traditional Conservative voters but only at the cost of further limiting the appeal of the Tory brand.